SQL內部聯接和計數

Question

我有3個表（用戶，項目，類似用戶）和2個sql查詢。 如何統一這兩個查詢？

SELECT item.userid, item.id, user.name FROM item 
INNER JOIN user ON item.userid = user.id 

SELECT userid,itemid, COUNT(*) AS `liked` FROM userlike
WHERE userid=9
GROUP BY itemid

我想知道特定用戶（9）是否喜歡該商品。

結果應該是這樣的

itemid userid name liked* (*whether 'user 9' liked this item or not)
1      7      foo  0
2      4      asd  1

謝謝

Answer 1

您要為此使用OUTER JOIN

SELECT i.id itemid, u.id userid, u.name, COALESCE(liked, 0) liked
  FROM item i JOIN user u
    ON i.userid = u.id LEFT  JOIN 
(
  SELECT itemid, COUNT(*) liked 
    FROM userlike 
   WHERE userid = 9
   GROUP BY itemid
) l
    ON i.id = l.itemid;

要么

SELECT i.id itemid, u.id userid, u.name, l.userid IS NOT NULL liked
  FROM item i JOIN user u
    ON i.userid = u.id LEFT  JOIN userlike l
    ON i.id = l.itemid
   AND l.userid = 9;

樣本輸出：

| ITEMID | USERID |  NAME | LIKED |
|--------|--------|-------|-------|
|      2 |      4 | user4 |     1 |
|      1 |      7 | user7 |     0 |

這是SQLFiddle演示

Answer 2

SELECT item.id, item.userid, user.name, userlike.liked
FROM item
JOIN user ON user.id = item.userid
JOIN userlike ON item.id = userlike.itemid
WHERE userlike.liked = 1
GROUP BY item.id

要么

SELECT item.id, item.userid, user.name, userlike.liked
FROM item
JOIN user ON user.id = item.userid
JOIN userlike ON item.id = userlike.itemid
WHERE COUNT(userlike.liked) >= 1
GROUP BY item.id

Answer 3

您不需要使用COUNT 。 您只需要知道在userlike表中是否存在特定項目和特定用戶的userlike ：

SELECT i.id as itemid, u.id as userid, u.name,
       case when ul.userid is null then 0 else 1 end as liked
FROM item i
INNER JOIN user u ON i.userid = u.id
LEFT OUTER JOIN userlike ul ON i.id = ul.itemid AND ul.userid=9
ORDER BY i.id